This invention is related generally to the quantification of porosity distribution in metal castings, and in particular to an improved quantitative metallographic methodology to accurately measure porosity and its distribution in cast components.
Porosity has long been recognized as an important detrimental factor affecting mechanical properties of casting components. The larger the pores, the lower the mechanical properties, where fatigue resistance has been shown to be particularly susceptible. Porosity location is also problematic, as for the same pore size, fatigue life is lower when the porosity is located closer to the stressed free surface. Aluminum and aluminum alloy castings—which are useful in automotive components such as engine blocks and related structures—are especially vulnerable to porosity.
In practice, porosity is commonly measured metallographically on as-polished planes, where fractographic measurements are usually conducted in a scanning electron microscope (SEM) on the fracture surfaces of such planed specimens. In one form, the measurement data is useful in daily casting operations for quality control, as well as for fatigue and other mechanical property predictions. It has been found that there is a significant difference between pore sizes measured on as-polished planes and the actual size of fracture surfaces in three dimensional (3D) space, where typically the pore size measured on an as-polished plane is the smaller of the two, often significantly so.
One reason for the difference between the traditional metallographic measurements and actual pore sizes in 3D can be attributed to the way in which the data from the metallographic image analysis (IA) or related computerized vision system is acquired and analyzed. Traditionally, the pore sizes measured on the as-polished planes are performed on a field-by-field basis, where the actual field size depends on camera resolution and magnification used. In many cases, a single pore can be located on the boundaries of several fields. As a result, some pores are partially measured in multiple fields of view which can result in the aforementioned under-measurement of the pore size. Another reason for the difference between the traditional metallographic measurements and actual pore sizes in 3D can be explained by the irregular-shaped pores that are measured when a two-dimensional (2D) plane is sectioned through an irregularly-shaped pore; in such circumstances, a single pore can be observed as several individual small pores on the section plane. Therefore, the ability of traditional metallographic measurements to provide accurate quantitative pore data is severely hampered. Concomitantly, to take such information and use it as input to a mathematical model (such as a fracture mechanics model) as a way to predict fatigue performance of the material will likely result in inaccuracies, especially in its tendency drastically overestimate the fatigue strength and the available life of the casting.
One alternative to provide quantitative pore data is through micro-focused X-ray computed tomography (CT); unfortunately, such an approach is expensive and time-consuming, and therefore not suited to a production-oriented environment. Another alternative includes the computational simulation and prediction of porosity; however, simplifying assumptions in the physics and complex casting process to enable the calculations and to reduce computational cost can result in relatively poor approximations of actual pore size.